1. Field of the Invention
The present invention relates to frequency conversion circuit. More particularly, the present invention relates to an automatic gain control circuit with low distortion.
2. Description of the Related Art
Frequency converters have been widely applied to cable television system to convert radio frequency (RF) signals to intermediate frequency (IF) signals. As shown in FIG. 1, a frequency converter generally includes three basic functional blocks: an RF amplifier 10, a mixer 12, and a local oscillator 14. Preferably, a circuit design with low noise is utilized to implement the RF amplifier 10 because the RF amplifier 10 is put at the front-end of the reception system. In FIG. 1, the RF amplifier 10 receives and amplifies an RF input signal and the local oscillator 14 generates an LO signal. Then, the mixer 12 combines the RF input signal and the LO signal to generate an IF signal.
The frequency bands for RF, image and LO signals entering the frequency converter applied to the TV tuner and the IF output signal generated by the frequency converter for a cable television system of 50-550 MHz are depicted in FIG. 5. The relationships among the IF, RF, LO and image frequencies for the frequency converter are as follows:
FREQ(IF)=FREQ(LO)-FREQ(RF input) PA1 FREQ(IF)=FREQ(IF image)-FREQ(LO)
A so-called Gilbert type mixer merging the RF amplifier 10 and the mixer 12 of FIG. 1 into a whole has been disclosed, the detailed circuit diagram of which is illustrated in FIG. 2. The Gilbert type mixer, a double balanced mixer, has excellent carrier suppression and low second order distortion. Although the double balanced mixer can be implemented by filters to achieve quite good carrier suppression, the demand on high-value inductors and high-value capacitors consumes a great deal of layout area not suited for use in integrated circuits.
FIG. 2 illustrates the detailed circuitry of a conventional Gilbert type mixer, wherein the mixer 12 (including four field-effect transistors 121, 122, 123, and 124) operates similarly to a standard four quadrant Gilbert variable transconductance multiplier. The RF amplifier 10 comprises a pair of field effect transistors 101 and 102 receiving input from the differential RF signal at terminals RF+ and RF-, amplifying it, and coupling the amplified differential RF input signal to mixer 12. Note that a pair of current sources 103 and 104 are connected in series with the associated field-effect transistors 101 and 102. The mixer 12 receives amplified differential RF input signal from the RF amplifier 10, differential LO signal via terminals LO+ and LO-, and mixes these signals to generate the differential IF output signal at terminals IF+ and IF-.
As shown in FIG. 2, an automatic gain control (AGC) circuit 20 is provided between the sources of the field-effect transistors 101 and 102 to adjust the gain of the receiver so that the IF output signal level remains substantially constant with varying RF input signal levels. The AGC circuit 20 comprises a pair of field-effect transistors 201 and 202 connected in series, a pair of resistors 203 and 204, and a voltage source 205. The voltage source 205 provides a voltage bias to the gates of the field-effect transistors 201 and 202 via the resistors 203 and 204, respectively. The use of two series-connected field-effect transistors 201 and 202 can render the linearity of AC signals quite well along with staged bias control.
However, the fact that the channel resistance R.sub.DS of the field-effect transistors 201 and 202 varies with the voltage source 205, as well as the RF small signals at the drain of the field-effect transistor 201 and the source of the field-effect transistor 202, gives rise to non-linear distortion.
Therefore, U.S. Pat. No. 5,563,545 discloses an improved AGC circuit 30 as shown in FIG. 3. In the drawing, the conventional AGC circuit 30 comprises a field-effect transistor 301, two capacitors 302 and 303, a resistor 304, and a voltage source 305. The field-effect transistor 301 are configured with its gate connected to the voltage source 305 through the resistor 304. The capacitor 302 is connected between the drain and gate of the field-effect transistor 301, whereas the capacitor 303 is connected between the source and gate of the field-effect transistor 301.
However, although the AGC circuit 30 of FIG. 3 can reduce non-linear distortion, additional inductors (not shown in the drawing) are required to connect the AGC circuit 30 and the respective sources of the field-effect transistors 101 and 102 in order to attenuate high-frequency noise.